Which of the following represents calgon?
A. $N{a_2}A{l_2}S{i_2}{O_8}$
B. $M{g_3}{(P{O_4})_2}$
C. $N{a_2}[N{a_2}{(P{O_3})_6}]$
D. $N{a_2}[M{g_2}{(P{O_3})_6}]$

Hint: Calgon is derived from the name calcium gone. Calgon are used for bathing and cleaning. It makes complexes with calcium ions and so is used as a water softener.

Complete step by step answer:
-Calgon is the composition of sodium hexametaphosphate or amorphous sodium polyphosphate. It removes calcium and magnesium salts from hard water and is used as water softener. They are used in soaps and detergents to enhance the action by preventing unwanted salts. Thus the formula of calgon is $N{a_2}[N{a_2}{(P{O_3})_6}]$ i.e. option C is the correct answer.
 $N{a_2}A{l_2}S{i_2}{O_8}$ : This is the molecular formula of Permutit. It is an inorganic salt of sodium aluminium silicate also known as zeolites. They are used as water softener by ion exchange method. They are normally used in big tanks over a layer of coarse sand.
 $M{g_3}{(P{O_4})_2}$ : Magnesium phosphate is an ionic compound. It is the compound of magnesium and phosphorus. As the name suggests it is used to overcome the deficiency of magnesium or phosphorus in the human body.
 $N{a_2}[M{g_2}{(P{O_3})_6}]$ : This is the product obtained when calgon removes magnesium from hard water. The magnesium present in hard water forms a complex with calgon thereby replacing the sodium with magnesium and as a result making the water soft.

So, the correct answer is “Option C”.

Note: The precipitated calcium and magnesium salt is a measure of the degree of hardness of water. Because of water softening properties it is used in washing machine cleaner in the form of powder, tablet and gel.